Being overweight in childhood, adolescence or during adulthood has traditionally been thought to increase the long-term risk of diabetes, hypertension, cardiovascular events and some cancers as well as reducing life expectancy. Our research has shown that this received wisdom may not be true for overweight older people.

Consider the expectation that a 72-year-old man should be as strong or as fast as a 30-year-old man. It is unrealistic but policy makers, health professionals and the population at large seem to expect that the body composition of a 72-year-old man should be the same as that of a 30-year-old man.

These disparate views of the world cannot be reconciled for the simple reason that body shape and composition change with time. As a result, the body of a 72-year-old will never be like the body of a 30-year-old.

What’s more, some of the physiological changes that take place with increasing age may be adaptive rather than harmful.

The amount and distribution of fat in older people is different from younger people and being different is not necessarily bad.

Our data shows that the 10-year risk of dementia is lowest for men in their 70s who are slightly overweight (but not obese). The risk is lower even than the risk for older men who have ‘normal’ weight.

So now we have a problem. If these findings are true, then health professionals may be giving bad advice to older people.

If an older person has a body mass index (BMI) of 27 and his doctor tells him to lose weight to bring his BMI down to 24, the doctor might be contributing to an increase rather than decrease that man’s health risks. (BMI is a commonly-used measure of the distribution of body fat.)

And if policy-makers state that a BMI lower than 25 is what we expect from people aged 70 years or over, they may be contributing to causing more harm than good to this population.

The results of our research suggest we may need to recalibrate the BMI ranges that are considered ‘underweight’, ‘normal’, ‘overweight’ and ‘obese’ for this older age group.

The results of future studies will give us greater certainty as to where to set those boundaries.

It is also important to note that the results were limited to men and may not apply to women. What we do know is that the body shape of women changes more markedly than the body shape of men with age.

Our group has published mortality data in 2010 showing survival is longest among slightly overweight women i.e. slightly overweight women had lower 10-year risk of death than normal weight or obese women.

In fact, these findings are very much like the results that we reported for men. So it would not be surprising if future studies find that overweight older women have lower risk of developing dementia than their normal weight counterparts, even though it is too early to say so definitively.